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(Mashable) -- We already know that Facebook is the web's biggest time sink. If you look at the average amount of time (according to Nielsen) users spend on the social network, Facebook is a clear winner over sites such as Google or Yahoo.

Now, according to comScore, Facebook is also first when it comes to the total amount of time users are spending on the site.

In August, U.S. web users spent 41.1 million minutes on Facebook, which was about 9.9 percent of their entire web-surfing time in that month.

In this same period, people spent 39.8 million minutes on all of Google's sites, and those include another huge online timesink -- YouTube.

comScore puts Yahoo in third place, with U.S. web users spending 37.7 mill... ion minutes on its sites, which was about 9.1 percent of their web surfing time in August.

The numbers are even more impressive when you consider that Facebook had just overtaken Yahoo in July, and in August last year U.S. web surfers had spent less than 5 percent of their online time on the social networking service.

Still, it hardly comes as a surprise: Facebook has been growing steadily in the last couple of years, and in July it announced it had over500 million active users.

If Facebook keeps growing, a year from now Google may find itself far behind Facebook when it comes to web users' minutes.

But does Facebook have room for growth?

Mark Zuckerberg predicts the site's userbase might even reach one billion. The number doesn't sound too far-fetched, given that Facebook still has room for international growth -- for example inChina and Russia.

Of course, comScore only counts users from the U.S., so the global picture is still blurry.

But the facts show that Facebook users spend a huge amount of time on the site, and it's a worrying stat for Google.

Google's many online properties (Gmail, Search and YouTube, to name a few) have vast influence and reach. But right now, without a large social networking property (Orkut doesn't count as serious competition to Facebook anymore), Google will have a hard time snatching users' time from Facebook's hands.